Circuit interrupter embodying polytetrafluoroethylene



Nov. 2l, 1950 A. P. srRoM r-:rAL 2,531,007

- CIRCUIT INTERRUPTER monnnc PoLmTRAFLUoRox-:mm Filed latch 20. 1947 2 Sheets-.Sheet 2 Fbef) 5w Fb l o e 4 e a /o /z impe/'e5 EMS, x/oop WITNESSES: INVENTOR 5 Patented Nov. 2l, i950 CIRCUIT INTERRUPTER EDIBODYING POLYTETBAFLUOBOETHYLENE Albert P. Strom and Leo J. Berberich, Pittsburgh,

Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 20, 1947, Serial No. 736,070

6 Claims.

This invention relates to circuit interrupters, and more specifically te circuit interrupting devices embodying arc confining and extinguishing members prepared from polytetrafiuoroethylene.

While a number of arc confining and extinguishing materials have been employed in circuit interrupters, certain prior art materials have been so outstanding in this service that they are often selected as a standard of comparison. In particular. horn or vulcanized fiber has exhibited such generally desirable characteristics over a wide range of conditions of use that it is commonly accepted as a standard of comparison for most materials. In certain applications, however, the heat of the arc being interrupted is so intense that even fiber erodes extensively, while other organic substances erode so much more that various undesirable features such as excessive carbonization, and excessive gas formation leading to back blast may occur. Therefore, the arc confining structures comprising horn fiber and other materials must be replaced at frequent intervals in order to maintain maximum emcieney in the breaker device.

Ithas been discovered that polytetrafluoroethylene has the unusual property of eroding to a markedly lesser extent that does horn fiber under the same conditions of use in the presence of electrical arcs. Tests have also indicated that polytetrailuoroethylene is exceptionally suitable for use as an arc confining and extinguishing material in any of a great variety of circuit interrupting devices.

'I'he object of this invention is to provide a circuit interruptor with an arc conning and interrupting structure comprising polytetrauoroethylene.

Other objects of the invention will in part be obvious and will in part appear hereinafter. For a better understanding of the nature and objects of the invention, reference should be had to the following description and drawings, in which:

Figure l is a fragmentary view in elevation partly in section of a magnetic type of circuit interrupter,

Fig. 2 is a fragmentary view in elevation partly broken of a compressed air type of circuit breaker.

Fig. 3 is a vertical cross-section of a portion of a restricted orifice gas blast circuit breaker,

Fig. 4 is a vertical cross-section through a fuse,

Pig. 5 is a graph plotting the relative erosion of polytetrafluoroethylene compared to fiber in a fuse up to 2000 amperes of current, while (Cl. ZOO-149) Fig. 6 plots the relative erosion volumes of fiber and polytetrafluoroethylene up to 12,000 amperes in a gas blast breaker.

According to the present invention, arc conning and extinguishing portions of circuit interrupters are prepared from polytetrauoroethylene. Polytetrafiuoroethylene has been found to erode less than any known organic material when subjected to the heat of an electrical arc. Polytetrafiuoroethylene is composed of carbon and fiuorinc, and has the generic formula lll-l i i When the polytetrafluoroethylene is subjected to high temperatures, as in an arc, the fluorine to carbon bond appears to be more stable than the carbon to carbon bond, and lower polymers result with very little free carbon being produced. The dielectric4 strength and other electrically insulating properties of polytetrafluoroethylene are outstanding.

For producing structures embodying polytetrafiuoroethylene, they may be machined from solid blocks of the polymer by using conventional metal or wood working tools or else the structures may be molded from the powdered polymer and sintered under heat and pressure. as disclosed in the Patents 2,406,127; 2,400,099; 2,400,094 and 2,230,654. In order to impart increased strength for supporting loads when heated to temperatures above 300 C., the polytetrafluoroethylene may be combined with glass fibers either loose or in the form of a. fabric by applying thereto powdered polytetrafiuoroethylene or interleaving films of polytetraf'luoroethylene with webs or sheets of glass fiber fabric and molding under heat and pressure into a homogeneous solid. After the members are formed, it may be desirable to anneal them under pressure at temperatures above 325 C. and below 500 C. te remove mechanical strains and to increase the homogeneity and density.

Members composed of polytetrafluoroethylene may be employed for arc splitters and for arcing chambers, fuse tubes, arc gaps, insulating supports adjacent the arc, and foi other structures in arc interrupting devices. For securing best results. both mechanically and electrically, the polytetrafiuoroethylene should comprise the major proportion of the member, particularly where the structure is intended to develop an arc-extinguishing gas. Less than 50% of the weight of the member may be composed of reinfibers or containing other fillers.

forcing fibers. such as asbestos or glass bers, or solid inorganic materials, such as titanium dioxide, silica, magnesium oxide, or the like. It will be appreciated that other gas-evolving materials, such as boric acid, ammonium borate or magnesium borate, as well as reinforcing solids, may be incorporated in the polytetrafluoroethylene.

Referring to Figure 1 of the drawings, there is illustrated a magnetic circuit interrupter l embodying the features of the present invention. 'Ihe circuit interrupter comprises a pair of arcing contacts I2 and I4 disposed within a casing comprising wall I6 which may be prepared from sheets or panels of polytetrafluoroethylene. A magnetic coil I8 adapted to be energized when the contacts I2 and I4 are separated is associated with a magnetic core and magnetic field pole plates 22 for drawing an arc upwardly into an arc-extinguishing structure comprising a plurality of splitter plates 24 of polytetrauoroethylene. or polytetrafluoroethylene reinforced with glass The arc is transferred on the separation of the contacts I2 and I4 to the arcing horns 26 and 28 and under the influence of the magnetic field between the field pole 22, it will be drawn up into the arcextinguishing splitter plate structure where the arc' is attenuated and extinguished by the action of small but highly effective amounts of arc-extinguishing gases generated from the polytetrafluoroethylene plates 24.

Experience with splitter plates of a great many organic insulating solids indicated that they evolved such relatively great quantities of gas that arcs would be blown back out o1' the plate structure in magnetic circuit interrupters without being interrupted satisfactorily. Also these organic materials carbonized excessively and were rendered poor insulators so that the operation became progressively poorer. Therefore, ceramic splitter plates are used commercially almost exclusively in magnetic circuit interrupters. However?, ceramic splitter plates have the disadvantage of low surface dielectric strength when heated by impinging electrical arcs and a large number of plates. with a bulky structure, are required for satisfactory operation. For example, a magnetic breaker capable of interrupting a maximum of 7500 volts 60 cycle current requires 5 a minimum of 48 plates of porcelain embodied in a casing 18 inches wide.

A considerably smaller number of splitter plates of polytetrailuoroethylene, due to its unusual properties, can be employed in the magnetic circuit interrupter. The amount of gas evolved on arcing is small and does not cause gas blow back. but, is sufdcient to improve the function of the apparatus. A further advantage resides in the considerable reduction in size of the splitter structure with a smaller magnetic coil and field pole plate structure as compared to that required for a ceramic plate structure.

For an illustration of a gas blast circuit breaker embodying polytetrauoroethylene, reference should be had to Fig. 2 of the\drawings. The gas blast circuit breaker 30 comprises a stationary contact member 32 and a movable contact arm 34 adapted to create an are when they \are separated in the process of interrupting a\circuit in which the contact members are embodied. The

end of the contact 34 enters through a slot It l in a throat 40 composed of polytetrafluoroethylene. A stream of compressed gas, such as air, is released into a conduit 35 at the time the contact member 34 is being separated from contact 32 and the gas blast blows the arc produced against the splitters 42 composed of polytetrafluoroethylene. The splitters 42 evolve a small amount of an arc-extinguishing gas which does not generate a back blast and thereby enables the are to be readily attenuated by being blown into the splitter structure. The low amount of gas generated from the polytetrafluoroethylene is readily blown away by the gas blast and the arc successfully extinguished when the current reaches zero.

In Fig. 3 of the drawings, there is illustrated a restricted orifice type of gas blast circuit breaker 50. In this breaker, the contact members 52 and 54 are separated from one another by an appropriate mechanism when it is desired to interrupt the circuit in which the interrupter is embodied. The contact members 52 and 54 are disposed in a tubular member '56 having a contracted diameter 58 resulting in a restricted orice 6I) where the contacts normally meet in closing the circuit. The orifice 58 and a reinforcing tube 62 may be both prepared from polytetrafluoroethylene, though for most purposes only the portion 58 bearing the restricted orifice need be made from polytetrafluoroethylene. 'Ihe remaining structure may be prepared from materials possessing a suitable physical strength or other desired characteristics.

In interrupting a circuit, the contact members 52 and 54 being in closed position at constricted throat S0, the contact members 52 and 54 are separated and a blast of air or other gas is directed along the tubular member 55. The blast of air is intensiiled in speed at the constricted throat so that its cooling action is increased. The heat of the arc causes arc-extinguishing gases to be evolved from the polytetrauoroethylene throat structure 58 and this also assists in cooling the arc. At current zero, the lengthened and cooled arc is readily extinguished. Since the shape and size of the throat 60 are critical, the use of polytetrafluoroethylene is particularly beneflcial in that so little of the material is eroded as compared to any known gas-evolving insulating material. 'Ihe low amount of gas generated permits reliable and successful operation.

Referring to the fuse III illustrated in Fig. 4 of the drawing, it comprises the strain wire 12 and a fusible wire "I4, both being connected to an upper terminal 'Il connectible to one portion of a circuit and a lower terminal 'I8 connected by a flexible conductor I2 to a lower terminal 84 connectible to the other portion of the circuit. The tube surrounding the wires I2 and I4 is prepared from polytetrafluoroethylene as described herein. Upon a sumciently high current passing through the wires 12 and 14, they will fuse, thereby creating an arc. The lower contact 'I5 will drop either by the presence of the gases developed by the heat of the arc alone, or by a spring (not shown) operating thereon to draw out the arc further. The polytetrauoroethylene in the tube 50 will evolve an arc-extinguishing gas thereby enabling the extinction of the arc. While an adequate quantity of arc-extinguishing gas will be evolved from the polytetrauoroethylene, the amount of erosion of the fuse walls will not be excessive and the fuse can be reused.

The various circuit interrupting structures shown in Figs. 1 to 4 are only exemplary and not exhaustive of the types of circuit interrupting structures to which the invention may be applied. In all these arc interrupting devices, polytetra.

uoroethylene may be employed as the arc conflning and arc-extinguishing structure disposed along the path of the arc for the purpose of providing an arc-extinguishing gas. It is well known that, particularly for heavy current interrupters, the amount of arc-extinguishing gas given on' may be so great that the function of the apparatus is not satisfactory. For example, the plates 24 (Fig. 1) and I2 (Fig. 2) may develop so much gas if an organic material other than polytetrafiuoroethylene be employed that the arc may be driven out of the splitters by a blow back gas blast, thereby reducing the eiectiveness and maximum current capacity of the interrupter. The outstanding advantage of polytetraiiuoroethylene resides in the totally unexpected reduction in erosion of the material by the heat of the arc.

Referring to Fig. 5 of the drawings, there is illustrated a graph comparing the relative erosion of hard ber and polytetrafluoroethylene in a iuse structure. It will be noted that the amount of polytetrauoroethylene eroded is less than of the volume eroded of hard ber at a current of 1200 amperes. This enables a fuse composed pf polytetrauoroethylene to be employed many times longer before it must be discarded as compared to a fiber fuse tube. Tests made with a gas blast circuit interrupter are illustrated in the graph of Fig. 6, wherein fiber and polytetrafluoroethylene splitters are compared in volume erosion. It will be apparent that at all times the erosion of polytetrafluoroethylene is less than half of the amount of ber eroded under the same conditions of current. This characteristic of polytetrauoroethylene renders it highly superior for use when exposed to arcing under all conditions.

Furthermore, polytetrailuoroethylene does not carbonize appreciablv as compared to any other organic material known. Tracking and creepage is much less than with other organic materials used for this purpose. This is ascribed to the i'act that it does not tend to break down between the fluorine and carbon atoms but rather to depolymeriae into uorocarbon compounds which are good insulators. No evidence of free iluorine formation has been found.

Since certain changes may be made in the above description and different embodiments of the invention can be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. An electric circuit interrupter comprising in combination, conductor means between which an arc is formed on interruption of the circuit, and arc confining and extinguishing means disposed along the path of the arc from which an arc extinguishing medium can be evolved due to the heat of the arc for extinguishing the arc, the arc connning and extinguishing means comprising polytetrailuoroethylene.

2. An electric circuit interrupter comprising, in combination, a pair of contact members disposed to produce an arc upon separation to interrupt the circuit, a chamber within which the contact members are arranged, and arc coni'lning and extinguishing means within the chamber arranged to evolve an arc extinguishing gas due to the heat of the arc, to enable extinction of the same, the arc confining and extinguishing means comprising polytetrafluoroethylene.

3. An electric circuit interrupter comprising, in combination, a pail` of contact members disposed to produce an arc upon separation to interrupt the circuit, a chamber within which the contact members are arranged, arc confining and extinguishing means comprising spaced members at one end of the chamber, and means for causing an arc to impinge on the spaced members and to pass into the spaces therebetween to evolve an arc extinguishing gas, the spaced members comprising polytetrafiuoroethylene.

4. An electric circuit interrupter comprising, in combination, a pair of contact members disposed to produce an arc upon separation to interrupt the circuit, a chamber within which the contact members are arranged, arc conning and extinguishing means comprising spaced members at oneend of the chamber, and means for directing a gas blast against the arc to cause it to impinge on the spaced members and to pass into the spaces therebetween to evolve an arc extinguishing gas, the spaced members comprising polytetrauoroethylene.

5. An electric circuit interrupter comprising, in combination, a pair of contact members disposed to produce an arc upon separation to interrupt the circuit, a chamber within which the contact members are arranged, arc confining and extinguishing means comprising spaced members at one end of the chamber, a magnetic member associated with the contact members to create a magnetic field during arcing to cause the arcto impinge on the spaced members and to pass into the spaces therebetween to evolve an arc extinguishing gas, the spaced members comprising polytetrauoroethylene.

6. In a fuse device, in combination, means adapted to fuse and form an arc on the occurrence of an overload in an electrical circuit embodying the fuse device, an arc confining and extinguishing structure associated with said means along the path of the arc from which an arc extinguishing medium can be evolved due to the heat of the arc for extinguishing it, the structure comprising polytetrafiuoroethylene.

ALBERT P. STROM. LEO J. BERBERICH.

REFERENCES CITED The following references are of record in the nie of this patent:

UNITED STATES PA'I'EN'I'S Number Name Date 2,073,565 Ruppel Mar. 9, 1937 2,142,840 Grosse Jan. 3, 1939 2,155,276 Korndorfer Apr. 18, 1939 2,157,832 Nowak et al. May 9, 1939 2,243,040 Ludwig et al May 20,' 1941 2,328,825 McMahon Sept. 7, 1943 OTHER REFERENCES Modern Plastics. June 1946. 

